Multi-Scale Mechanical Property Evaluation of Soft Materials
Dr. Bill Knowlton
Comprehensive evaluation of the biomechanical properties associated with the initiation of joint-related diseases is needed to promote a more thorough understanding of cartilage degeneration mechanisms. For instance, probing the biomechanical properties, from a multi-scale perspective (cartilage matrix to the collagen fibrils), could provide insights into the failure modes of degenerative diseases. A multi-scale approach enabling high resolution quantitative mechanical property maps is needed to develop cartilage structure-property relationships. In this study, the quantitative mechanical property measurement capabilities were inversely related to the spatial resolution of the technique. The techniques include vertical nanoindentation with loads ranging in the 1000s µN, atomic force microscopy (AFM) cantilevered nanoindentation with loads ranging in the 10s µN, and AFM-based torsional harmonics imaging (HMX) with loads ranging in the 0.01s µN. Prior to probing biomaterials, the accuracy of the three systems were evaluated using a model material system, polycarbonate (PC) and poly(methyl methacrylate) (PMMA). The polymers were selected for multiple reasons: (i) to avoid substrate effects, (ii) to minimize the influence of heterogeneities inherent to biomaterial composites and (iii) because of their similar material properties (elastic modulus and viscoelasticity). Using the polymer material systems, it was determined that the AFM and HMX measurements are within 15% of the expected elastic modulus as measured with vertical indentation. The predominant source of the variation is most likely from inaccurate tip area calculations, viscoelastic effects and force calibration. With the variation between the three systems established, AFM and HMX-based mechanical property measurements were made on collagen, a component of cartilage with extensive mechanical property data available for comparison. The results for collagen were within the range of published values of 1- 3 GPa. HMX coupled with the AFM and vertical indentation techniques enable high spatial resolution mechanical property maps that can potentially be used to study the initiation of degenerative diseases.